U.S. patent application number 12/861834 was filed with the patent office on 2011-02-24 for vamos transmission schemes.
Invention is credited to David Ben-Eli, Tomer Goshen, Efrat Isack, Paul S. Spencer, Amir Winstok.
Application Number | 20110044299 12/861834 |
Document ID | / |
Family ID | 43086480 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110044299 |
Kind Code |
A1 |
Spencer; Paul S. ; et
al. |
February 24, 2011 |
VAMOS transmission schemes
Abstract
A method includes assigning a communication channel for
simultaneous transmission to at least first and second mobile
communication terminals using a Time-Division Multiplexing (TDM)
communication protocol. Upon determining that data is to be
transmitted over the communication channel to both the first and
the second mobile communication terminals, the data is transmitted
at a first power level that is defined for transmission to the
first mobile communication terminal as part of the simultaneous
transmission. Upon determining that the data is to be transmitted
over the communication channel only to the first mobile
communication terminal, the data is transmitted at a second power
level that is defined for transmission only to the first mobile
communication terminal and is lower than the first power level.
Inventors: |
Spencer; Paul S.; (Modiin,
IL) ; Winstok; Amir; (Ganai-Tikva, IL) ;
Isack; Efrat; (Elkana, IL) ; Goshen; Tomer;
(Kfar Saba, IL) ; Ben-Eli; David; (Modiin,
IL) |
Correspondence
Address: |
D. Kligler I.P. Services LTD
P.O. Box 25
Zippori
17910
IL
|
Family ID: |
43086480 |
Appl. No.: |
12/861834 |
Filed: |
August 24, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61236434 |
Aug 24, 2009 |
|
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61308735 |
Feb 26, 2010 |
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Current U.S.
Class: |
370/336 |
Current CPC
Class: |
H04W 72/121 20130101;
H04W 52/262 20130101; H04W 72/0446 20130101; H04W 52/346 20130101;
H04W 52/20 20130101; H04W 52/36 20130101; H04W 52/143 20130101 |
Class at
Publication: |
370/336 |
International
Class: |
H04J 3/00 20060101
H04J003/00 |
Claims
1. A method, comprising: assigning a communication channel for
simultaneous transmission to at least first and second mobile
communication terminals using a Time-Division Multiplexing (TDM)
communication protocol; upon determining that data is to be
transmitted over the communication channel to both the first and
the second mobile communication terminals, transmitting the data at
a first power level that is defined for transmission to the first
mobile communication terminal as part of the simultaneous
transmission; and upon determining that the data is to be
transmitted over the communication channel only to the first mobile
communication terminal, transmitting the data at a second power
level that is defined for transmission only to the first mobile
communication terminal and is lower than the first power level.
2. The method according to claim 1, comprising predefining the
first and second power levels.
3. The method according to claim 2, wherein assigning the
communication channel comprises assigning first and second
orthogonal quadrature sub-channels for transmission to the first
and second mobile communication terminals, respectively, and
wherein predefining the first power level comprises setting a
projection of the simultaneous transmission on the first quadrature
sub-channel to have the first power level.
4. The method according to claim 3, wherein transmitting the data
at the first power level comprises modulating the data for the
first and the second mobile communication terminals onto respective
first and second quadrature components of a Quadrature Phase Shift
Keying (QPSK) signal, and wherein transmitting the data at the
second power level comprises modulating the data onto the first
quadrature sub-channel using Gaussian Minimum Shift Keying
(GMSK).
5. The method according to claim 2, wherein predefining the first
power level comprises setting the first power level so as to meet a
target error rate at the first mobile communication terminal during
the simultaneous transmission, and wherein predefining the second
power level comprises setting the second power level so as to meet
the target error rate at the first mobile communication terminal
during transmission only to the first mobile communication
terminal.
6. The method according to claim 1, wherein transmission to the
second mobile communication terminal is carried out in a
Discontinuous Transmission (DTX) mode, and wherein determining that
the data is to be transmitted only to the first mobile
communication terminal comprises determining a time period of the
DTX mode in which there is no active speech for transmission to the
second mobile communication terminal.
7. The method according to claim 1, wherein determining that the
data is to be transmitted only to the first mobile communication
terminal comprises determining that the communication channel is
currently allocated only to the first mobile communication
terminal.
8. Apparatus, comprising: a transmitter, which is configured to
transmit signals using a Time-Division Multiplexing (TDM)
communication protocol to at least first and second mobile
communication terminals; a resource scheduler, which is configured
to assign a communication channel for simultaneous transmission to
at least the first and second mobile communication terminals; and a
controller, which is configured to: cause the transmitter to
transmit data at a first power level that is defined for
transmission to the first mobile communication terminal as part of
the simultaneous transmission, when the data is to be transmitted
over the communication channel to both the first and the second
mobile communication terminals, and, to cause the transmitter to
transmit the data at a second power level, which is defined for
transmission only to the first mobile communication terminal and is
lower than the first power level, when the data is to be
transmitted over the communication channel only to the first mobile
communication terminal.
9. The apparatus according to claim 8, wherein the controller is
configured to predefine the first and second power levels.
10. The apparatus according to claim 9, wherein the resource
scheduler is configured to assign first and second orthogonal
quadrature sub-channels for transmission to the first and second
mobile communication terminals, respectively, and wherein the
controller is configured to predefine the first power level by
setting a projection of the simultaneous transmission on the first
quadrature sub-channel to have the first power level.
11. The apparatus according to claim 10, wherein controller is
configured to transmit the data at the first power level by
modulating the data for the first and the second mobile
communication terminals onto respective first and second quadrature
components of a Quadrature Phase Shift Keying (QPSK) signal, and to
transmit the data at the second power level by modulating the data
onto the first quadrature sub-channel using Gaussian Minimum Shift
Keying (GMSK).
12. The apparatus according to claim 8, wherein the controller is
configured to set the first power level so as to meet a target
error rate at the first mobile communication terminal during the
simultaneous transmission, and to set the second power level so as
to meet the target error rate at the first mobile communication
terminal during transmission only to the first mobile communication
terminal.
13. The apparatus according to claim 8, wherein transmission to the
second mobile communication terminal is carried out in a
Discontinuous Transmission (DTX) mode, and wherein the controller
is configured to determine that the data is to be transmitted only
to the first mobile communication terminal by determining a time
period of the DTX mode in which there is currently no active speech
for transmission to the second mobile communication terminal.
14. The apparatus according to claim 8, wherein the controller is
configured to determine that the data is to be transmitted only to
the first mobile communication terminal by determining that the
communication channel is currently allocated only to the first
mobile communication terminal.
15. A method, comprising: allocating a communication channel,
comprising a sequence of time slots, for transmissions to at least
first, second and third mobile communication terminals, such that
each time slot carries a respective transmission addressed to up to
two of the mobile communication terminals; predefining a slot
pairing schedule, which assigns a first subset of the time slots
for the transmissions to the first and second mobile communication
terminals and assigns a second subset of the time slots for the
transmissions to the first and third mobile communication
terminals; and transmitting signals to at least the first, second
and third mobile communication terminals in accordance with the
predefined slot pairing schedule.
16. The method according to claim 15, wherein transmitting the
signals comprises transmitting only to the second mobile
communication terminal during at least a first time slot in the
slot pairing schedule, and only to the third mobile communication
terminal during at least a second time slot in the slot pairing
schedule.
17. The method according to claim 16, wherein transmitting the
signals comprises transmitting to the first mobile communication
terminal in a Discontinuous Transmission (DTX) mode in which at
least the first and second time slots are assigned to the first
mobile communication terminal but do not carry speech signals for
the first mobile communication terminal.
18. The method according to claim 16, wherein allocating the
communication channel comprises assigning no communication terminal
other than the second mobile communication terminal to the first
time slot, and assigning no communication terminal other than the
third mobile communication terminal to the second time slot.
19. Apparatus, comprising: a transmitter, which is configured to
transmit signals to at least first, second and third mobile
communication terminals; and a processor, which is configured to
allocate a communication channel, comprising a sequence of time
slots, for transmissions to at least the first, second and third
mobile communication terminals, such that each time slot carries a
respective transmission addressed to up to two of the mobile
communication terminals, to predefine a slot pairing schedule,
which assigns a first subset of the time slots for the
transmissions to the first and second mobile communication
terminals, and which assigns a second subset of the time slots for
the transmissions to the first and third mobile communication
terminals, and to cause the transmitter to transmit signals to at
least the first, second and third mobile communication terminals in
accordance with the predefined slot pairing schedule.
20. The apparatus according to claim 19, wherein the processor is
configured to cause the transmitter to transmit only to the second
mobile communication terminal during at least a first time slot in
the slot pairing schedule, and only to the third mobile
communication terminal during at least a second time slot in the
slot pairing schedule.
21. The apparatus according to claim 20, wherein the processor is
configured to operate the first mobile communication terminal in a
Discontinuous Transmission (DTX) mode in which at least the first
and second time slots are assigned to the first mobile
communication terminal but do not carry speech signals for the
first mobile communication terminal.
22. The method according to claim 20, wherein the processor is
configured to assign no communication terminal other than the
second mobile communication terminal to the first time slot, and to
assign no communication terminal other than the third mobile
communication terminal to the second time slot.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application 61/236,434, filed Aug. 24, 2009, and U.S.
Provisional Patent Application 61/308,735, filed Feb. 26, 2010,
whose disclosures are incorporated herein by reference.
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates generally to communication
systems, and particularly to methods and systems for assigning
communication channels to users.
BACKGROUND
[0003] In some communication networks, signals are transmitted
simultaneously to multiple users over the same radio resource. For
example, some proposed Global System for Mobile communications
(GSM) EDGE Radio Access Network (GERAN) configurations support a
feature that is referred to as Multi-User Reusing-One-Slot (MUROS),
also known as Voice services over Adaptive Multi-user channels on
One Slot (VAMOS). MUROS/VAMOS defines a scheme in which two users
are multiplexed in the same time slot.
[0004] MUROS is described, for example, by the Third Generation
Partnership Project (3GPP) in "Technical Specification Group GERAN;
Circuit Switched Voice Capacity Evolution for GSM/EDGE Radio Access
Network (GERAN); (Release 8)," TR 45.914, version 8.1.0, May, 2009,
which is incorporated herein by reference. Assignment of users to
MUROS/VAMOS channels is described in "Technical Specification Group
GSM/EDGE Radio Access Network; Multiplexing and multiple access on
the radio path (Release 9)," TS 45.002, version 9.1.0, September,
2009, which is incorporated herein by reference.
[0005] The description above is presented as a general overview of
related art in this field and should not be construed as an
admission that any of the information it contains constitutes prior
art against the present patent application.
SUMMARY
[0006] An embodiment that is described herein provides a method, in
which a communication channel is assigned for simultaneous
transmission to at least first and second mobile communication
terminals using a Time-Division Multiplexing (TDM) communication
protocol (e.g., a GSM protocol). Upon determining that data is to
be transmitted over the communication channel to both the first and
the second mobile communication terminals, the data is transmitted
at a first power level that is defined for transmission to the
first mobile communication terminal as part of the simultaneous
transmission. Upon determining that the data is to be transmitted
over the communication channel only to the first mobile
communication terminal, the data is transmitted at a second power
level that is defined for transmission only to the first mobile
communication terminal and is lower than the first power level.
[0007] In an embodiment, the method includes predefining the first
and second power levels. In a disclosed embodiment, assigning the
communication channel includes assigning first and second
orthogonal quadrature sub-channels for transmission to the first
and second mobile communication terminals, respectively, and
predefining the first power level includes setting a projection of
the simultaneous transmission on the first quadrature sub-channel
to have the first power level.
[0008] In another embodiment, transmitting the data at the first
power level includes modulating the data for the first and the
second mobile communication terminals onto respective first and
second quadrature components of a Quadrature Phase Shift Keying
(QPSK) signal, and transmitting the data at the second power level
includes modulating the data onto the first quadrature sub-channel
using Gaussian Minimum Shift Keying (GMSK). In yet another
embodiment, predefining the first power level includes setting the
first power level so as to meet a target error rate at the first
mobile communication terminal during the simultaneous transmission,
and predefining the second power level includes setting the second
power level so as to meet the target error rate at the first mobile
communication terminal during transmission only to the first mobile
communication terminal.
[0009] In some embodiments, transmission to the second mobile
communication terminal is carried out in a Discontinuous
Transmission (DTX) mode, and determining that the data is to be
transmitted only to the first mobile communication terminal
includes determining a time period of the DTX mode in which there
is no active speech for transmission to the second mobile
communication terminal. In an embodiment, determining that the data
is to be transmitted only to the first mobile communication
terminal includes determining that the communication channel is
currently allocated only to the first mobile communication
terminal.
[0010] There is additionally provided, in accordance with an
embodiment that is described herein, apparatus including a
transmitter, a resource scheduler and a controller. The transmitter
is configured to transmit signals using a Time-Division
Multiplexing (TDM) communication protocol to at least first and
second mobile communication terminals. The resource scheduler is
configured to assign a communication channel for simultaneous
transmission to at least the first and second mobile communication
terminals. The controller is configured to cause the transmitter to
transmit data at a first power level that is defined for
transmission to the first mobile communication terminal as part of
the simultaneous transmission, when the data is to be transmitted
over the communication channel to both the first and the second
mobile communication terminals. When the data is to be transmitted
over the communication channel only to the first mobile
communication terminal, the controller is configured to cause the
transmitter to transmit the data at a second power level, which is
defined for transmission only to the first mobile communication
terminal and is lower than the first power level.
[0011] There is also provided, in accordance with an embodiment
that is described herein, a method in which a communication
channel, including a sequence of time slots, is allocated for
transmissions to at least first, second and third mobile
communication terminals, such that each time slot carries a
respective transmission addressed to up to two of the mobile
communication terminals. A slot pairing schedule is predefined,
which assigns a first subset of the time slots for the
transmissions to the first and second mobile communication
terminals and assigns a second subset of the time slots for the
transmissions to the first and third mobile communication
terminals. Signals are transmitted to at least the first, second
and third mobile communication terminals in accordance with the
predefined slot pairing schedule.
[0012] In some embodiments, transmitting the signals includes
transmitting only to the second mobile communication terminal
during at least a first time slot in the slot pairing schedule, and
only to the third mobile communication terminal during at least a
second time slot in the slot pairing schedule. In an embodiment,
transmitting the signals includes transmitting to the first mobile
communication terminal in a Discontinuous Transmission (DTX) mode
in which at least the first and second time slots are assigned to
the first mobile communication terminal but do not carry speech
signals for the first mobile communication terminal. In a disclosed
embodiment, allocating the communication channel includes assigning
no communication terminal other than the second mobile
communication terminal to the first time slot, and assigning no
communication terminal other than the third mobile communication
terminal to the second time slot.
[0013] There is further provided, in accordance with an embodiment
that is described herein, apparatus that includes a transmitter and
a processor. The transmitter is configured to transmit signals to
at least first, second and third mobile communication terminals.
The processor is configured to allocate a communication channel,
including a sequence of time slots, for transmissions to at least
the first, second and third mobile communication terminals, such
that each time slot carries a respective transmission addressed to
up to two of the mobile communication terminals, to predefine a
slot pairing schedule, which assigns a first subset of the time
slots for the transmissions to the first and second mobile
communication terminals, and which assigns a second subset of the
time slots for the transmissions to the first and third mobile
communication terminals, and to cause the transmitter to transmit
signals to at least the first, second and third mobile
communication terminals in accordance with the predefined slot
pairing schedule.
[0014] The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a block diagram that schematically illustrates a
wireless communication network that uses VAMOS, in accordance with
an embodiment that is described herein;
[0016] FIGS. 2 and 3 are flow charts that schematically illustrate
methods for downlink transmission over a VAMOS channel, in
accordance with embodiments that are described herein; and
[0017] FIGS. 4A-4B and 5-6 are diagrams that schematically
illustrate slot pairing schedules, in accordance with embodiments
that are described herein.
DETAILED DESCRIPTION OF EMBODIMENTS
[0018] In some communication systems, a Base Station (BS) transmits
signals to two or more Mobile Stations (MSs) over a single shared
communication channel. An example of such a system is the Global
System for Mobile communications (GSM) EDGE Radio Access Network
(GERAN) network, which supports Voice services over Adaptive
Multi-user channels on One Slot (VAMOS). In VAMOS systems, the BS
transmits signals to two MSs simultaneously on the same
communication channel by transmitting two respective quadrature
sub-channels, e.g., using Alpha Quadrature Phase Shift Keying
(.alpha.-QPSK) modulation. The communication channel typically
comprises a sequence of time slots that are transmitted on a given
Radio Frequency (RF) channel.
[0019] In some scenarios, however, the BS has data to transmit to
only one MS over a given physical communication channel. (A
definition of a physical channel is given, for example, in section
5 of the TS 45.002 specification, cited above.) For example, one of
the MSs assigned to the communication channel may operate in a
Discontinuous Transmission (DTX) mode in order to reduce
interference. In this mode, the BS transmits encoded speech to the
MS only when active speech is present, and only minimal signaling
is transmitted to the MS during periods of low speech activity.
When using DTX, the BS occasionally transmits signals only to a
single MS, even though two MSs are assigned to the communication
channel. As another example, the BS may transmit to a single MS
when no other MS is assigned to the same communication channel,
either temporarily or permanently.
[0020] In an embodiment, the BS transmits simultaneously to two MSs
using a two-bits-per-symbol modulation scheme such as .alpha.-QPSK.
However, when transmitting to a single MS, the BS typically uses a
one-bit-per-symbol modulation scheme such as Gaussian Minimum Shift
Keying (GMSK). (In alternative embodiments, other modulation
schemes may be used.) The performance of one-bit-per-symbol
modulation is often considerably higher than the performance of
modulation schemes that use two or more bits per symbol, in terms
of the Signal-to-Noise (S/N) or Signal-to-Interference (C/I) ratio
that is required to meet a certain error probability. Therefore,
when the BS transmits only to a single MS over a shared
communication channel, this MS enjoys reception conditions that are
often considerably better than those required to meet the specified
reception performance.
[0021] Embodiments that are described herein provide methods and
systems that exploit the improved reception conditions during
transmission to a single MS in order to improve the overall system
performance. In some embodiments, the BS transmits to a given MS at
different transmit power levels, depending on whether the
transmission to the MS is part of a simultaneous transmission
(together with transmission to another MS simultaneously on the
same communication channel) or not. The BS sets a first power level
for transmission to the MS as part of the simultaneous transmission
(e.g., sets the transmit power level of the quadrature sub-channel
that carries the transmission to the MS in question). The BS sets a
second transmit power level, lower than the first power level, for
transmission only to the given MS (e.g., using GMSK).
[0022] Since the modulation used for transmitting only to the given
(single) MS is typically more robust than the modulation used for
simultaneous transmission, the second (lower) transmit power level
is sufficient for meeting the specified reception performance.
Reducing the transmit power level when transmitting only to the
given MS reduces the interference caused by the BS to neighboring
BSs and MSs. In a typical implementation, the BS sets both transmit
power levels so as to meet the same reception performance
requirement (e.g., a maximum frame error rate of 1%).
[0023] In some embodiments that are described herein, the BS
assigns up to four MSs that use half-rate speech coding to a single
shared physical communication channel. The communication channel
comprises a sequence of time slots. Each time slot is used for
transmission to one or two of the MSs using one-bit-per-symbol or
two-bits-per-symbol modulation, as explained above. Because of the
half-rate speech coding, each MS is assigned half of the time slots
of the communication channel, on average. In these configurations
too, there may be occasions in which the BS transmits only to a
single MS on a given time slot. For example, when one of the four
MSs operates in DTX and has no active speech to receive, in an
embodiment, the MS that is paired with this MS on the same time
slot receives one-bit-per-symbol signals and typically has
considerably improved reception performance.
[0024] In some embodiments, the BS pairs the MSs onto the
communication channel in accordance with a predefined pairing
schedule, which specifies which MSs are to be paired in each time
slot. In particular, the pairing schedule alternates the pairing
over time. In other words, when pairing at least first, second and
third MSs onto the communication channel, the pairing schedule
specifies that the first MS is paired with the second MS in at
least one time slot, and with the third MS in at least one other
time slot.
[0025] The alternating pairing helps to distribute the improved
performance provided by the one-bit-per-symbol modulation among the
MSs, rather than have a specific MS enjoy this benefit.
Distributing the performance improvement among multiple MSs
improves the performance of the worst-performing MS, with little or
no degradation in the performance of the other MSs.
[0026] FIG. 1 is a block diagram that schematically illustrates a
wireless communication network 20, in accordance with an embodiment
that is described herein. Network 20 comprises a base Station (BS)
24, which communicates with mobile communication terminals 28, also
referred to as Mobile Stations (MSs). In the present example,
network 20 comprises a GERAN network that supports MUROS or VAMOS
(the terms MUROS and VAMOS are used interchangeably herein). The
disclosed techniques, however, are not limited to VAMOS, and are
applicable to networks that use any other suitable Time-Division
Multiplexing (TDM) based communication protocol.
[0027] In an embodiment, MSs 28 comprise, for example, cellular
phones, communication-enabled mobile computing devices, cellular
adapters for mobile computing devices or any other suitable type of
communication terminal. Although FIG. 1 shows a single BS and four
MSs for the sake of clarity, real-life networks typically comprise
multiple BSs and multiple MSs.
[0028] BS 24 comprises a BS processor 32, which manages the BS
operation. In particular, the BS processor controls the power level
at which the BS transmits to the different MSs, and the assignment
of MSs to communication channels, using methods that are described
in detail below. BS processor 32 may perform such functionalities
(e.g., channel assignment and control of transmission power) using
separate units within the BS processor. Alternatively, a single
processing unit may perform any suitable subset of these functions,
or even all functions. The BS processor accepts downlink data for
transmission to the MSs, and produces downlink signals for
transmission. The downlink signals are provided to a BS transmitter
(BS TX) 36, which transmits the signals using a BS antenna 40 to
MSs 28.
[0029] In each MS 28, the downlink signals are received by a MS
receiver (MS RX) 48 via a MS antenna 44. A MS processor 52
reproduces the downlink data that was transmitted to the MS from
the BS. (The internal MS structure is shown in FIG. 1 only for the
MSs denoted MS1 and MS2, for the sake of clarity. The other MSs
typically have a similar internal structure.)
[0030] In some embodiments, BS processor 32 comprises a transmit
(TX) power setting unit 56, which sets the transmit power of the BS
on the different VAMOS sub-channels. In some embodiments, BS
processor 32 comprises a resource scheduler 60, which assigns MSs
to VAMOS communication channels. Example techniques for transmit
power setting and for channel assignment are described in detail
below.
[0031] The system, BS and MS configurations shown in FIG. 1 are
simplified example configurations, which are depicted solely for
the sake of conceptual clarity. In alternative embodiments, any
other suitable system BS and/or MS configuration can be used.
System, BS and MS elements that are not necessary for understanding
the disclosed techniques have been omitted from the figure for the
sake of clarity. For example, since the disclosed techniques are
mainly concerned with downlink transmission, BS and MS elements
that are related to the uplink (e.g., a MS transmitter and a BS
receiver) have been omitted from the figure.
[0032] The different elements of BS 24 and MS 28, including
processor 32 (and in particular units 56 and 60), transmitter 36,
receiver 48 and/or processor 52, may be implemented using dedicated
hardware, such as using one or more Application-Specific Integrated
Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other
hardware devices. Alternatively, some BS and/or MS elements may be
implemented using software executing on general-purpose hardware,
or using a combination of hardware and software elements. In some
embodiments, processor 32 and/or processor 52 comprise programmable
processors that are programmed in software to carry out the
functions described herein, although they too may be implemented on
dedicated hardware. The software may be downloaded to the
processors in electronic form, over a network, for example, or it
may, alternatively or additionally, be provided and/or stored on
non-transitory tangible media, such as magnetic, optical or
electronic memory. In some embodiments, some or all of the elements
of MS 28 may be fabricated in a chip-set.
[0033] In an embodiment, BS 24 transmits the downlink signals using
a Time-Division Multiplexing (TDM) communication protocol such as a
3GPP GERAN protocol. In accordance with the protocol, multiple
communication channels (also referred to as resources) are
interleaved with one another in the time domain. In the example
embodiments described herein, BS 24 transmits one or more Radio
Frequency (RF) carriers (also denoted RF channels), which in some
embodiments hop in frequency. On each RF carrier, the BS transmits
a sequence of Time-Division Multiple Access (TDMA) frames, each
frame comprising eight time slots. The sequence of time slots
comprising a given time slot in each TDMA frame on a given RF
carrier (e.g., the second time slot in each TDMA frame) is regarded
as a communication channel or resource. In alternative embodiments,
any other suitable arrangement of traffic channels can also be
used.
[0034] When using VAMOS, resource scheduler 60 in BS 24 assigns a
given communication channel to up to four MSs. When using full-rate
speech coding, for example, a given communication channel is
assigned to one or two MSs. When using half-rate speech coding, up
to four MSs can be assigned the same communication channel. Hybrid
scenarios that mix full-rate and half-rate speech coding are also
feasible.
[0035] On a given time slot, the BS uses one of two possible
modulation schemes, depending on whether it transmits to a single
MS or to two MSs simultaneously. When transmitting to a single MS
on a given time slot, the BS transmits to the single MS using a
modulation scheme that modulates one bit per symbol, such as
Gaussian Minimum Shift Keying (GMSK). When transmitting to two MSs
simultaneously on a given time slot, the BS transmits to the two
MSs using a modulation scheme that modulates two bits per symbol.
Typically, this modulation scheme comprises a quadrature modulation
scheme, which modulates the traffic for the two MSs on two
respective mutually-orthogonal quadrature sub-channels.
[0036] The two-bits-per-symbol modulation scheme may comprise, for
example, Quadrature Phase Shift Keying (QPSK). In another
embodiment, the BS uses a variant of QPSK, in which the two
quadrature sub-channels are given different gains. This variant is
sometimes referred to as .alpha.-QPSK, wherein a denotes the ratio
between the gains of the two quadrature sub-channels. The
description that follows assumes that the BS transmits using GMSK
when transmitting to a single MS, and using .alpha.-QPSK when
transmitting to two MSs simultaneously.
[0037] In some practical cases, a certain communication channel
(resource) is defined as a VAMOS channel that is able to support
simultaneous transmission to two MSs, but the BS nevertheless
transmits only to a single MS during at least some of the time
slots of this channel. For example, one or both MSs assigned to
this channel may operate in a Discontinuous Transmission (DTX) mode
in order to reduce interference to other BSs or MSs. In the DTX
mode, the BS transmits in the time slots of the channel only if
there is active speech to be transmitted. In GERAN systems, for
example, when using full-rate speech coding, only twelve TDMA
frames out of every 104 TDMA frames would contain transmitted time
slots if the BS has no active speech to transmit. For half-rate
speech coding, eight TDMA frames out of every 104 TDMA frames would
contain transmitted time slots if the BS has no active speech to
transmit. (One half-rate channel uses only fifty-two out of the 104
TDMA frames, while the other fifty-two TDMA frames are used by the
other half-rate channel.) In a continuous transmission mode, by
contrast, the BS transmits in every time slot regardless of speech
activity.
[0038] When one or both MSs that are assigned to a certain channel
use DTX, some time slots of that channel will contain transmission
for only one of the MSs. The likelihood and distribution of such
time slots depend on the speech activity statistics of the MS
users. In an example scenario, active speech is present for a given
MS during approximately 50% of the time. Speech activity and
inactivity periods alternate at a rate that is on the order of
seconds. In other scenarios, time slots of a certain VAMOS
communication channel may carry transmissions to only one MS
because the second MS has not yet initiated communication (e.g.,
handed-off) to this channel. Further alternatively, a VAMOS
communication channel may temporarily carry transmission to only a
single MS for any other reason.
[0039] When transmitting to a single MS on a given time slot, the
BS reverts to GMSK modulation, as opposed to .alpha.-QPSK
modulation that is used for simultaneous transmission to two MSs.
This scenario may occur, for example, during intermittent DTX
periods or when there is no other MS currently assigned to the time
slot. In the description that follows, time slots in which the BS
transmits to a single MS using GMSK are referred to as GMSK time
slots, and time slots in which the BS transmits to a single MS
using .alpha.-QPSK are referred to as .alpha.-QPSK time slots. Both
the GMSK time slots and the .alpha.-QPSK time slots belong to the
same VAMOS communication channel.
[0040] The performance of GMSK modulation is considerably higher
than the performance of .alpha.-QPSK modulation, in terms of the
Signal-to-Noise (S/N) or Signal-to-Interference (C/I) ratio that is
required to achieve a certain error probability. Therefore, unless
accounted for, the GMSK time slots enjoy considerably better
conditions than the .alpha.-QPSK time slots. In some embodiments,
BS 24 uses the improved conditions of the GMSK time slots to
improve the overall system performance.
[0041] In some embodiments, TX power setting unit 56 in BS 24 sets
the transmit power that is used for transmission to a given MS to
different power levels, depending on whether the same time slot is
also used simultaneously for transmitting to another MS. In a
typical embodiment, unit 56 sets a first power level for
transmitting to the given MS if the time slot in question is an
.alpha.-QPSK slot (i.e., if the same time slot is also used for
transmitting to another MS). If the time slot is a GMSK slot (i.e.,
if transmission on this time slot is only to the given MS), unit 56
sets a second power level that is lower than the first power
level.
[0042] The reduction in power level in the GMSK time slots reduces
the interference caused by the BS to neighboring cells, and
therefore improves the overall system performance. Since GMSK
modulation is considerably more resilient to noise and interference
than .alpha.-QPSK modulation, suitable reduction in power level
causes little or no degradation in reception of the GMSK signal at
the MS.
[0043] When comparing the transmit power levels used in the GMSK
slots and the .alpha.-QPSK slots, the comparison should not
consider the total power of the .alpha.-QPSK, but only the power of
the quadrature sub-channel that is used for transmitting to the
given MS. When referring to simultaneous transmission to two MSs on
a VAMOS channel, the term "power level that is used for
transmission to a MS as part of the simultaneous transmission"
refers only to the power level on the quadrature sub-channel that
is used for transmission to that MS, and not to the overall
transmit power on the VAMOS channel. In an .alpha.-QPSK slot, the
power level on a given quadrature sub-channel can be defined as the
power level of the projection of the .alpha.-QPSK signal on that
sub-channel. When switching between .alpha.-QPSK transmission and
GMSK transmission (e.g., when one MS goes in and out of DTX), the
total transmit power on the channel changes inherently because one
of the quadrature sub-channels is switched on and off. This sort of
change is not considered a change in the transmit power level
toward the other MS.
[0044] For a given MS, unit 56 can set the first power level (used
in GMSK slots) and the second power level (used in .alpha.-QPSK
slots) to any suitable values and according to any suitable
criteria. In some embodiments, unit 56 sets both power levels to
values that meet a target error rate (e.g., Frame Error Rate--FER).
In an example embodiment, the target FER at the MS is defined as
1%, for both GMSK and .alpha.-QPSK transmissions. In this
embodiment, unit 56 sets the transmit power level during the
.alpha.-QPSK slots such that the corresponding quadrature
sub-channel is received by the MS at a FER that meets the target
FER. During GMSK time slots that are used for transmitting to this
MS, unit 56 reduces the transmit power level to a value that meets
the target FER for GMSK.
[0045] When setting the first and second power levels to meet the
same target FER, the difference between the two power levels
depends on the value of .alpha., i.e., on the power ratio between
the two quadrature sub-channels of the .alpha.-QPSK signal. For
.alpha.=-8 dB, for example, the transmit power level used for
transmitting to the MS having the weaker quadrature sub-channel can
be reduced by approximately 4 dB during the GMSK slots, while still
meeting the 1% target FER. This 4 dB reduction in power enables
considerable reduction in the interference to neighboring
cells.
[0046] FIG. 2 is a flow chart that schematically illustrates a
method for downlink transmission over a VAMOS channel, in
accordance with an embodiment that is described herein. The method
description refers to a scenario in which BS 24 has assigned two
MSs denoted MS1 and MS2 to a certain VAMOS channel.
[0047] The method begins with unit 56 in BS 24 defining (or
accepting a definition of) a target FER, at a target FER definition
operation 70. At a first power level definition operation 74, unit
56 defines a first power level for use in transmission to MS1 as
part of simultaneous VAMOS transmission to MS1 and MS2 (e.g., in
.alpha.-QPSK slots). At a second power level definition operation
78, unit 56 defines a second power level for use in transmission
only to MS1 (e.g., in GMSK slots). Both transmit power levels are
chosen so as to meet the target FER defined at operation 70
above.
[0048] At a certain point in time, BS processor 32 accepts downlink
data for transmission, at an input operation 82. At least some of
the downlink data is to be transmitted to MS1. Possibly, some of
the downlink data is to be transmitted to MS2. In and embodiment,
when preparing to transmit the downlink data, BS processor 32
checks whether data is to be transmitted only to MS1, or to both
MS1 and MS2, at a checking operation 86. Data may be available only
for MS1, for example, when MS2 operates in DTX mode and the BS
currently has no active speech to transmit to MS2.
[0049] If the BS processor concludes that data is available for
transmission to both MS1 and MS2, the BS transmits to MS1 and MS2
simultaneously using .alpha.-QPSK, at a simultaneous transmission
operation 90. For the simultaneous transmission, unit 56 sets the
transmit power level of the quadrature sub-channel that is used for
transmitting to MS1 to the first power level defined at operation
74. The method loops back to operation 82 above in which the BS
processor accepts additional downlink data for transmission.
[0050] If, on the other hand, the BS processor concludes that data
is to be transmitted only to MS1, the BS transmits only to MS1
using GMSK, at a GMSK transmission operation 94. For this
transmission, unit 56 sets the transmit power level of the GMSK
signal that is used for transmitting to MS1 to the second power
level defined at operation 78. The method then loops back to
operation 82 above.
[0051] In some embodiments that are described herein, BS 24
allocates a given VAMOS channel (resource) for up to four half-rate
MSs. The VAMOS channel comprises a sequence of time slots
(typically the k.sup.th time slot in each TDMA frame on a given RF
carrier). As with full-rate MSs, each time slot serves up to two
MSs. When using half-rate speech coding, however, each MS uses on
average only one time slot every two TDMA frames. In some cases, a
given time slot on the VAMOS channel carries transmission only to a
single MS (e.g., when the other MS assigned to this time slot
operates in DTX and has no active speech to receive). When a given
time slot carries transmission only to a single MS, the BS
transmits during this time slot using GMSK instead of
.alpha.-QPSK.
[0052] As explained above, the performance of GMSK modulation
typically is considerably better than the performance of
.alpha.-QPSK modulation. Therefore, a MS that receives GMSK signals
(because the BS does not transmit to any other MS on the same time
slot) will enjoy considerably better conditions than a MS that
receives a quadrature sub-channel of a .alpha.-QPSK signal.
[0053] When assigning four half-rate MSs to a VAMOS channel, the BS
divides the four MSs into pairs, and transmits to each pair
simultaneously over a subset of the time slots of the VAMOS
channel. The BS may divide the MSs into pairs according to various
criteria. Although it is possible in principle to define a fixed
division of the MSs into pairs, this sort of division is likely to
cause only selected MSs to enjoy the superior conditions of GMSK
reception.
[0054] Consider, for example, four half-rate MSs denoted MS1 . . .
MS4 that are assigned a certain VAMOS channel. Assume, for example,
that MS1 operates in a DTX mode, and that MS2 . . . MS4 operate in
a continuous transmission mode. Assume also that the BS pairs MS1
constantly with MS2 and MS3 constantly with MS4. In this scenario,
whenever MS1 has no active speech to receive, the BS transmits to
MS2 using GMSK. MS3 and MS4, on the other hand, always receive
.alpha.-QPSK signals and never receive GMSK signals, since they are
constantly paired together. Thus, the benefit of having MS1 operate
in DTX is enjoyed only by MS2 and never by MS3 or MS4.
[0055] In some embodiments that are described herein, BS 24 changes
the division of MS1 . . . MS4 into pairs from time to time. As a
result, the benefit of having one or more MSs operate in DTX is
distributed among the MSs. In some embodiments, BS processor 32
predefines (or accepts a definition of) a slot pairing schedule,
which specifies the alternation in pairing MSs over time. In an
example embodiment, the slot pairing schedule spans multiple TDMA
frames (e.g., one or more traffic multi-frames, each comprising
twenty-six TDMA frames). For each TDMA frame, the predefined
schedule defines which of the four MSs are paired together. The
schedule defines alternating pairing. In other words, in a certain
subset of the time slots of the schedule MS1 is paired with MS2,
and in another subset of the time slots in the schedule MS1 is
paired with MS3. When using this sort of pairing schedule, the
improved conditions that are caused by MS1 operating in DTX are
enjoyed by both MS2 and MS3.
[0056] FIG. 3 is a flow chart that schematically illustrates a
method for downlink transmission over a VAMOS channel, in
accordance with an embodiment that is described herein. The method
begins with BS 24 assigning a certain VAMOS channel to at least
three MSs that use half-rate speech coding, at an assignment
operation 100. In the present example, the BS assigns the VAMOS
channel to four MSs denoted MS1 . . . MS4.
[0057] Resource scheduler 60 (FIG. 1) in the BS predefines (or
accepts a definition of) a slot pairing schedule for the given
VAMOS channel, at a schedule definition operation 104. The slot
pairing schedule defines the pairing of MS1 . . . MS4 on the
different time slots of the VAMOS channel. The pairing schedule
alternates over time, such that a given MS (e.g., MS1 in the
present example) is paired with different MSs at different times.
When using such alternating pairing, if a certain MS operates in
DTX, the performance benefit of being paired with this MS is
distributed among the other three MSs. Several examples of slot
pairing schedules are shown in FIGS. 4A, 5 and 6 below. BS 24 then
transmits downlink data to MS1 . . . MS4 over the VAMOS channel in
accordance with the predefined slot pairing schedule, at a
transmission operation 108.
[0058] Although the description above refers to performance
benefits caused by a certain MS operating in DTX, the alternating
pairing techniques described herein are useful in other
circumstances, as well. For example, consider a scenario in which
the BS allocates only three MSs using half-rate speech coding in
VAMOS mode on the channel. In such a scenario, two MSs are paired
together (and therefore receive .alpha.-QPSK signals from the BS)
while the third MS is not paired (and therefore receives GMSK
signals). In some embodiments, the BS assigns time slots to the
three MSs according to the predefined pairing schedules described
herein, in which the pairing alternates over time. When using this
technique, at least some of the MSs receive GMSK signals during
some of the time slots and .alpha.-QPSK signals during some of the
time slots. In these embodiments too, the performance benefit
resulting from GMSK transmission is distributed among the three MSs
rather than enjoyed by only one of them.
[0059] FIG. 4A is a diagram that schematically illustrates an
example slot pairing schedule, in accordance with an embodiment
that is described herein. The schedule assigns time slots 110 on a
given VAMOS channel for downlink transmission to MS1 . . . MS4,
such that the pairing of MSs alternates over time. The example slot
pairing schedule of FIG. 4A spans one GERAN traffic multi-frame,
i.e., twenty-six TDMA frames (120 mS). The BS typically applies the
schedule periodically in subsequent traffic multi-frames.
[0060] Two of the twenty-six TDMA frames (the 13.sup.th and
26.sup.th TDMA frames in the multi-frame) comprise a Slow
Associated Control Channel (SACCH), and therefore do not carry
speech data transmissions to the MSs. These TDMA frames are marked
"S" in the figure. For the remaining twenty-four TDMA frames in the
traffic multi-frame, the schedule comprises respective twenty-four
time slots 110 that carry transmissions to MS1 . . . MS4, one
transmission in each TDMA frame. These time slots are marked "T" in
the figure.
[0061] The top row of the diagram shows the MS assignment to one of
the quadrature sub-channels (denoted "SUBCH0"). The bottom row of
the diagram shows the MS assignment to the other quadrature
sub-channel (denoted "SUBCH1"). The four MSs are shown using
different shading patterns. As can be seen in the figure, the
pairing of MSs alternates over time. For example, the MS that is
represented by a dotted pattern is paired with one MS (represented
by a diagonal pattern) during six time slots 110, and with a
different MS (represented by a horizontal pattern) during other six
time slots 110 in the multi-frame.
[0062] FIG. 4B is a diagram that schematically illustrates the slot
pairing schedule of FIG. 4A when transmission to one of the MSs
uses DTX, in accordance with an embodiment that is described
herein. In the present example, the BS transmits to only three of
the four MSs that are allocated to the VAMOS channel in question.
Therefore, half of time slots 110 carry simultaneous .alpha.-QPSK
signals addressed to two MSs, and half of the time slots carry GMSK
signals addressed to a single MS. This scenario occurs, for
example, when one MS operates in DTX and the BS has no active
speech to transmit to it during this multi-frame. This scenario
also occurs when only three MSs are allocated to the physical
channel, either temporarily or permanently.
[0063] As can be seen in the figure, because of the alternating
pairing defined by the pairing schedule, BS 24 transmits GMSK
signals to both MSs of SUBCH1 with equal likelihood. Thus, the
performance benefit of receiving GMSK signals (as opposed to
.alpha.-QPSK signals) is distributed between the two MSs of
SUBCH1.
[0064] The slot pairing schedule of FIGS. 4A and 4B can also be
viewed in a different manner: The assignment of MSs to time slots
on SUBCH0 alternates in every TDMA frame. On SUBCH1, however, the
time slot assignment alternates only once every two TDMA frames. In
other words, the two quadrature sub-channels alternate the
assignment of MSs to time slots with different cycles. This
difference in cycles between the two sub-channels causes the
pairing of MSs to change over time. Note that since in SUBCH0 the
assignment of MSs to time slots alternates in every TDMA frame,
SUBCH0 can be used to communicate with legacy MSs that do not
support the scheduling patterns described herein.
[0065] FIGS. 5 and 6 are diagrams that schematically illustrate
slot pairing schedules, in accordance with alternative embodiments
that are described herein. In particular, FIG. 6 shows a pairing
schedule in which the pairing alternates at the boundary of the
13.sup.th TDMA frame, i.e., in the middle of the traffic
multi-frame. During each half of the multi-frame, the pairing is
fixed.
[0066] The pairing schedules of FIGS. 4A, 4B, 5 and 6 are depicted
purely by way of example. In alternative embodiments, any other
suitable pairing schedule can also be used. For example, in the
examples above the pairing schedule is synchronized to the traffic
multi-frame and does not change from one multi-frame to another.
This configuration simplifies the implementation in the BS and in
the MS. Alternatively, however, schedules that change from one
traffic multi-frame to another are also contemplated. Typically,
the disclosed pairing schedules have no effect on codec selection
(selection between half-rate and full-rate speech coding) or on
allocation of full-rate MSs. As noted above, legacy MSs that do not
support the alternate-pairing schedules described herein can be
assigned to SUBCH0. Nevertheless, such legacy MSs also enjoy the
performance benefits provided by the alternating pairing
scheme.
[0067] Although the embodiments described herein mainly address
VAMOS schemes that transmit to one or two MSs in each time slot,
the methods and systems described herein can also be used, mutatis
mutandis, in transmission schemes that transmit to three or more
MSs per time slot using higher-order modulation. For example, the
scheme of FIG. 2 above can be generalized to high order modulation
by applying different power levels for transmission to a given MS
as a function of the number of MSs being served by the same
physical channel. In particular, the BS transmits to a single MS at
a lower power level than the power level used for transmitting to
that MS simultaneously with transmissions to other MSs on the same
physical channel, using any high-order modulation. Thus, when using
the disclosed techniques, transmit power for transmission to a
given MS can be graduated so that the transmit power is increased
as a function of the number of MSs to which simultaneous
transmissions are made over a shared channel.
[0068] It is noted that the embodiments described above are cited
by way of example, and that the present invention is not limited to
what has been particularly shown and described hereinabove. Rather,
the scope of the present invention includes both combinations and
sub-combinations of the various features described hereinabove, as
well as variations and modifications thereof which would occur to
persons skilled in the art upon reading the foregoing description
and which are not disclosed in the prior art.
* * * * *